Zinc phosphate layers can be formed on various metal surfaces, e.g. iron, steel, zinc alloy plated steel, aluminum or aluminum-plated steel by a phosphating treatment with aqueous solutions based on zinc phosphate. In addition to zinc and phosphoric acid, the phosphating solution may contain other cations and anions and the solutions are applied by spraying, dipping or spraying and dipping. The resulting zinc phosphate coatings afford protection against corrosion, improve the adhesion of paint, reduce sliding friction, facilitate cold working and provide electrical insulation.
In addition to the phosphating treatment proper a phosphating process includes various pretreating and after-treating stages. It is essential to clean the metal surfaces. This is generally effected by means of alkaline or acid cleaners and has the result that oils, greases, oxides and adhering solid particles are removed from the metal surface. If cleaning is effected by means of mild alkaline cleaners, it will be possible, in principle, to combine the cleaning with the activation of the metal surface. But the activation is suitably effected in a separate process step after the cleaning.
By the activation of the metal surface it is desired to ensure that a zinc phosphate layer which is as finely crystalline as possible will be formed in the shortest possible phosphating time. For this reason a criterion of the effect of the activating agent is the minimum phosphating time. The ability to form even finely crystalline zinc phosphate coatings can be determined from the weight of the layer or from micrographs prepared by a scanning electron microscope.
Activating agents based on titanium(IV) phosphate have proved particularly satisfactory in practice. Titanium(IV) phosphates will be formed by the reaction of aqueous solutions of titanium(IV) salts with soluble phosphates or phosphoric acid. But products having activating properties will be obtained only under special production conditions, which have been described, for example, in U.S. Pat. Nos. 2,310,239 and 2,456,947, which contain exact information regarding the nature and concentration of the raw material, temperature and pH value range during the production. Even if constant reaction conditions are maintained, however, the action in application technology will fluctuate from charge to charge.
A disadvantage involved in the use of activating agents based on titanium(IV) phosphate is that deionized water must be used to prepare the activating baths because the activating baths will be destabilized by alkaline earth metal ions which are contained in tap water and contribute to its hardness. Such alkaline earth metal ions may also be introduced into the activating bath by spent rinsing water.
In order to avoid the detrimental action of alkaline earth metal ions and thus to avoid a destabilization of the activating bath it has been proposed in DE-A-37 31089 to admix cation-exchanging zeolites having a primary particle diameter below 3 .mu.m to the activating titanium phosphate. Alternatively the activating baths may be improved in that, as described in EP-8-180523, phosphonic acid as a complexing agent is added to the activating bath so that industrial waste water may be used to prepare the bath. Besides, the presence of phosphonic acid allegedly causes the zinc phosphate coating to have an extremely fine crystallinity. Various phosphonic acids, however, have the severe disadvantage that they will poison the phosphating baths even in concentrations of a few mg/l and phosphonic acid introduced into the phosphating bath from the activating bath may render the phosphating bath unusable within a very short time.
In accordance with DE-A-38 14 28 poly(aldehydecarboxylic acids) in substoichiometric amounts are added as complexing agents for titanium(IV) during the preparation of activating titanium(IV) phosphates so that titanium phosphates having a small particle size (below 200 .mu.m) will mainly be formed and allegedly increase the activity of the activating agent. In that case too an introduction of the poly(aldehydecarboxylic acids) into the phosphating bath may involve considerable disadvantages.
The possibility to improve the stability of the activating baths against the hardness of water by suitable additives and the possibility to improve the quality as is reflected by the service life of the activating baths and by the crystallinity of the zinc phosphate coating formed in the succeeding stage by the use of complexing agents has been discussed previously. However, considerable disadvantages are involved in the measures by which said improvements can be achieved. On disadvantages, which is particularly involved in the use of complexing agents, is that they will poison the phosphating bath and will render the treatment of the waste water more difficult because they cause heavy metals to be dissolved or to be kept in solution.